Machine vision refers to methods and apparatus that are used to provide imaging-based automatic inspection and analysis for applications such as automatic inspection, process control and robot guidance in industry. Employing the proper lighting is critical in creating a reliable, repeatable machine vision application.
Currently, common light sources applied in a machine vision inspection system include halogen, fluorescent, xenon, LED and OLED lamps. Since LEDs have many advantages over other light sources (e.g., long life, low power consumption, fast response time, durability, etc.), LEDs have been widely used in lighting systems for machine vision inspection systems and have gradually replaced other types of light sources, becoming the most commonly utilized technology in machine vision inspection systems.
LED lighting systems are available in numerous form factors including area, line, ring and spot lights. As an example, a line scan application is defined as a machine vision application that utilizes a one-dimensional (1D) line scan camera, and a line light is a lighting system which provides a 1D line of illumination. The most common types of line scan applications are in web manufacturing of various materials (e.g., paper, foil, film, etc.) and in high resolution imaging of larger discrete parts. Such web manufacturing applications typically operate at very high speeds and often operate “24/7” (i.e., 24 hours a day, 7 days a week).
Machine vision systems can only create high quality images if the lighting used to inspect an object clearly illuminates the elements of the object which is being analyzed.
For many applications, the object under inspection is moving at high speed past the camera of the machine vision system, and appropriate lighting apparatus is required in order to obtain the necessary image of the object being inspected. In these situations, LED lights are often “overdriven” by strobe (or pulse) control so as to increase the intensity of the light emitted by the LED light source for a short, defined period of time.
LED lighting manufacturers typically offer LEDs which have a range of different wavelengths for different applications. Aside from the more traditional product offerings of white, red, green, and blue LEDs, ultraviolet (UV) and infrared (IR) LEDs are becoming increasingly popular for performing certain vision inspection tasks. These include use in systems where the materials to be inspected may exhibit fluorescence or where operating personnel must be shielded from bright visible spectrum LED illumination. In other cases, illumination products that combine LEDs to provide multiple wavelengths are being used to reduce the number of inspection stations required in order to inspect multiple aspects of complex objects. For example, a Red-Green-Blue (RGB) LED light head (i.e., LED light source) can produce a dynamic range of colors by independently adjusting the intensity values of the Red, Green, and/or Blue LEDs. This feature allows the operator to dynamically change the color of the light emitted by the LED light head and thus increase the contrast attainable with the machine vision system without deploying multiple light sources (e.g., without providing multiple line lights). Additionally, when all three channels (Red, Green, Blue) are color controlled, the LED light head can produce white light.
In many situations, it is desirable that a line light be capable of operating in single wavelength mode or in multi-wavelength mode (i.e., a mode which produces a combination of different wavelengths). Furthermore, it is frequently also desirable that the line light be capable of adjusting the intensities of the individual LEDs, modifying the strobing functionality of the line light and/or providing the ability to introduce delay signals to the line light (e.g., for synchronizing the line light with cameras).
It is also desirable that the line light utilize a modular design. This modularity provides two important advantages. First, it makes the line light easily serviceable. Among other things, when a given line light module needs to be replaced, that line light module should be easy to remove and replace by a non-technical person. Furthermore, the replacement line light module should then automatically configure itself via software controls to operate in conjunction with the remainder of the line light. Second, modularity makes it possible to provide the line light in varying lengths. To this end, it should be appreciated that the line light should, preferably, be configurable to varying lengths. Different original equipment manufacturers (OEMs) have different length requirements for their line lights, and even individual OEMs have systems with multiple length requirements for their line lights. Providing a modular line light would simplify the production of a line light of variable length.
Further issues relating to the mechanical, optical and electronic aspects of a modular LED line light will hereinafter be discussed.
1. Mechanical Issues
For many machine vision systems, OEMs build highly customized systems for their customers. As a result, the length requirements of their LED line lights vary from system to system. It would be desirable to provide an LED line light that is modular in design, for example a line light capable of having lengths ranging from 100 mm to 5 m, so that the requirements of various customers can be easily met without having to redesign existing line lights. Furthermore, an optimally-designed modular line light system would allow easily replaceable line light modules to be installed in a fast and easy manner.
In many line light applications, the operating environment is harsh. By way of example but not limitation, the line light may be located in an outdoor environment where it may be subjected to significant wind, rain and variations in temperature. By way of further example but not limitation, the line light may be located in an indoor environment where it may be subjected to caustic work environments such as those found on food production lines. In operation, the line light may need to be enclosed in a housing to protect it from such harsh environments. This requirement of a protective housing typically limits the amount of space available for LED driver boards and for fans to cool the LEDs.
Modular line lights also create an issue with respect to the control of individual line light modules within the modular line light. More particularly, these line light modules generally require a unique identification number so that the controller board for the line light can send the correct control signals to each line light module. For line lights operating within a protective enclosure (e.g., in a harsh environment), it is generally difficult to manually assign a unique identification (ID) number to the line light module, due to the difficulty in physically accessing the line light module. Furthermore, there can be issues in the field when some line light modules are replaced or moved to a different location within the modular line light. If the user fails to assign (or reassign) the unique ID numbers for each of the line light modules, the line light will not work as the user expects. Therefore, it would be desirable if auto-enumeration of each line light module in the line light (i.e., auto assignment of each unique ID number for each line light module) is carried out automatically when turning on the line light. In other words, it would be desirable if each line light module has its unique ID number assigned or reassigned to it automatically when turning on the modular LED line light.
In many OEM systems, space is a major constraint. LED line lights need to be designed to minimize space requirements while still ensuring that the reliability and performance of the line light are not compromised.
Furthermore, in keeping with the modular design of the line light, it is desirable to be able to easily replace the LED driver board for a given line light module should the LED driver board fail. Locating the LED driver board within the interior of a line light module generally makes replacement of the LED driver board impractical. By locating the LED driver board for a line light module on the exterior of the line light module, individual LED driver boards can be easily replaced. The LED driver board can be removed and tested simply by removing the LED driver board from the exterior of the line light module, without disrupting any of the internal components of the line light module (e.g., without disrupting alignment of the LED array contained within the line light module). Finally, as noted above, for some applications, it is necessary to enclose the line light in a protective housing to protect it from harsh work environments. This puts a further restriction on the location and size of the LED driver boards for the line light modules.
In addition, the provision of a protective housing about the line light complicates the use of cooling fans to cool the LEDs of the line light. To this end, it would be advantageous for a modular line light to be liquid-cooled (e.g., water-cooled) since it is difficult to utilize cooling fans when the line light is enclosed within a protective housing. In this respect it will be appreciated that even though the LEDs used by the line light may not be of high intensity, some cooling is necessary in order to regulate temperature during operation of the line light.
2. Optical Issues
Some OEM machine vision systems measure objects of varying sizes with the same system. It is, therefore, generally desirable for the “working distance” of the machine vision system to have a relatively wide range, for example, it can be desirable to provide line cameras and line lights that have a working distance across a range of 500 mm. For many commercially available line lights, there is a significant fall-off in the intensity of the line of light at the edges of the line of light emitted from the line light. See FIG. 1. Commercially-available line lights typically exhibit a fall-off in intensity of as much as 30% at the edges of the line of light. To counteract this light fall-off effect, most machine vision systems use line lights that extend beyond the edges of the region being scanned (e.g., that extend beyond the edges of conveyor belts), thereby ensuring that the area under inspection (e.g., the object on the conveyor belt) is relatively uniform. See FIG. 2. However, in many OEM machine vision systems, there is no room to extend the line light beyond the region being scanned, e.g., to extend the line light beyond the width of the conveyor belt. See FIG. 3. In this situation, it is important that the line light provide uniform illumination along the length of the line of light so as to ensure proper inspection of an object.
A further issue arises when lasers are utilized alongside LED lighting in OEM machine vision systems. Due to the simultaneous use of lasers and LED lighting in the machine vision system, “background” light emitted by the line light needs to be minimized so that measurements being taken by the lasers are not distorted. In most “off the shelf” line lights, some stray light will be emitted at the leading and trailing edges of the line of light emitted by the line light. See FIG. 4. In other words, the line of light produced by the line light will not have uniform intensity across its thickness; it will have stray light at the beginning and ends of its thickness dimension. It is important in machine vision systems that stray light emitted by the line lights does not affect the laser data being recorded by the machine vision camera. Thus, it is important that the line light emit an intensity profile which minimizes stray light across its thickness dimension. See FIG. 5.
3. Electronic Issues
As discussed earlier, it is desirable for many OEMs to provide a multi-wavelength line light (i.e., a line light capable of emitting light at multiple wavelengths). The entire line light needs to operate in single wavelength mode or in multi-wavelength mode (i.e., providing light of multiple wavelengths). Adjustment of the intensities of the individual LEDs, strobing functionality and the ability to input delay signals to the line light (e.g., to provide synchronization with line cameras) is required. These line lights need to offer current sensing, voltage sensing, control of the light intensity and control of the strobing functionalities, all within a small form factor and within tight cost constraints.
As will hereinafter be discussed, the present invention addresses these issues.